Mass function of haloes: scale invariant models

Press-Schechter theory gives a simple, approximate functional form of the mass function of dark matter haloes. Sheth and Tormen (ST) refined this mass function to give an improved analytical fit to results of N-body simulations. These forms of the halo mass function are universal (independent of cosmology and power spectrum) when scaled in suitable variables. Using large suites of LCDM N-body simulations, studies in the last few years have shown that this universality is only approximate. We explore whether some of the deviations from universality can be attributed to the power spectrum by computing the mass function in N-body simulations of various scale-free models in an Einstein-de Sitter cosmology. This choice of cosmology does not introduce any scale into the problem. These models have the advantage of being self-similar, hence stringent checks can be imposed while running these simulations. This set of numerical experiments is designed to isolate any power spectrum dependent departures from universality of mass functions. We show explicitly that the best fit ST parameters have a clear dependence on power spectrum. Our results also indicate that an improved analytical theory with more parameters is required in order to provide better fits to the mass function.Comment: 8 pages, four figure

An investigation of the compressive strength of PRD-49-3/Epoxy composites

The development of unidirectional fiber composite materials is discussed. The mechanical and physical properties of the materials are described. Emphasis is placed in analyzing the compressive behavior of composite materials and developing methods for increasing compressive strength. The test program for evaluating the various procedures for improving compressive strength are reported

A bacterial gene-drive system efficiently edits and inactivates a high copy number antibiotic resistance locus.

Gene-drive systems in diploid organisms bias the inheritance of one allele over another. CRISPR-based gene-drive expresses a guide RNA (gRNA) into the genome at the site where the gRNA directs Cas9-mediated cleavage. In the presence of Cas9, the gRNA cassette and any linked cargo sequences are copied via homology-directed repair (HDR) onto the homologous chromosome. Here, we develop an analogous CRISPR-based gene-drive system for the bacterium Escherichia coli that efficiently copies a gRNA cassette and adjacent cargo flanked with sequences homologous to the targeted gRNA/Cas9 cleavage site. This "pro-active" genetic system (Pro-AG) functionally inactivates an antibiotic resistance marker on a high copy number plasmid with ~ 100-fold greater efficiency than control CRISPR-based methods, suggesting an amplifying positive feedback loop due to increasing gRNA dosage. Pro-AG can likewise effectively edit large plasmids or single-copy genomic targets or introduce functional genes, foreshadowing potential applications to biotechnology or biomedicine

Formation rates of Dark Matter Haloes

We derive an estimate of the rate of formation of dark matter halos per unit volume as a function of the halo mass and redshift of formation. Analytical estimates of the number density of dark matter halos are useful in modeling several cosmological phenomena. We use the excursion set formalism for computing the formation rate of dark matter halos. We use an approach that allows us to differentiate between major and minor mergers, as this is a pertinent issue for semi-analytic models of galaxy formation. We compute the formation rate for the Press-Schechter and the Sheth-Tormen mass function. We show that the formation rate computed in this manner is positive at all scales. We comment on the Sasaki formalism where negative halo formation rates are obtained. Our estimates compare very well with N-Body simulations for a variety of models. We also discuss the halo survival probability and the formation redshift distributions using our method.Comment: 30 pages, 9 figure